traxxas 2.5 = .15
Traxxas 3.3 = .18
OS 18 CVR = .18 = same size as traxxas 3.3
OS 21tm = .21 = More powerful replacement for the traxxas 3.3
OS engines are very good engines and have been very reputable for 30+ years. I personally have been running os for over 20 years and would recommend an os engine over a traxxas.
if your looking for a reliable, easy to tune, comparable engine, OS 18CVR. its technically the same size as the trx 3.3 and a little faster then the trx 3.3. The os21 is a huge power house over a trx 3.3. Still a great engine but puts a lot more power out. or you could run carb restictors in it to keep the power down.